1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and the like, and more particularly, it relates to an image forming apparatus having a sheet re-feeding apparatus for re-feeding a sheet to permit formation an image on a second surface of the sheet having a first surface on which an image was already formed.
2. Related Background Art
In the past, image forming apparatuses such as copying machines, printers, facsimiles and the like, having a both-face mode for forming images on both surface of a single sheet are already known.
FIG. 7 shows an entire construction of a conventional image forming apparatus having a both-face mode.
At left and right parts of a lowermost portion of a main body 2 of an image forming apparatus 1, there are provided deck cases (sheet stacking means) 3a, 3b disposed side by side. For example, these deck cases 3a, 3b are capable of containing of about 1500 small size sheets (such as A4 size sheets) which are frequently used.
In FIG. 7, sheets P in the right deck case 3a are supplied and separated at a sheet supply portion 31a, and the separated sheet is directed into a registration introduction path (sheet convey path) 5b through a sheet feed path 5d.
In FIG. 7, sheets P in the left deck case 3b are supplied and separated at a sheet supply portion 31b, and the separated sheet is directed into the registration introduction path 5b through a sheet feed path 5a.
The sheet P introduced into the registration introduction path 5b is passed through an introduction path 5c and is conveyed to an image forming portion (transfer portion) 11 through a pair of registration rollers 10.
The sheet P on which an image was formed (transferred) in the image forming portion 11 is conveyed to a fixing device 15 through a convey portion 13. In the fixing device 15, heat and pressure are applied to the sheet to fix a toner image to the sheet P.
A pair of sheet discharge rollers 17 are disposed at a downstream side of the fixing device 15. A flapper 16 for switching the sheet P having a first surface on which the image was formed toward a reversing introduction path 18 is disposed between the fixing device and the pair of sheet discharge rollers 17.
In a both-face mode, after the image was formed on the first surface of the sheet, the sheet P is guided into the reversing introduction path 18 by the flapper 16, and the reversing a front surface and a rear surface of the sheet is effected in a reversing portion 19, and then, the sheet is conveyed to a re-feed path 20. When the sheet P is entered into the re-feed path 20, the sheet was turned over.
The sheet P is conveyed through the re-feed path 20. When a leading end of the sheet P reaches a sheet sensor 20b, the fact that the sheet P reached an end of the re-feed path 20 is detected by the sheet sensor 20b.
As a result, command is emitted from a control means (not shown) to a drive means (not shown) of the re-feed path 20, so that the drive means is temporarily stopped to temporarily stop the sheet P at a waiting position 20p spaced apart from the sheet sensor 20b by several ten millimeters in a downstream direction. Thereafter, the sheet P having the first surface on which the image was formed is fed into the registration introduction path 5b again through a junction 5bj to the registration introduction path 5b at a predetermined timing after a new sheet P was supplied from the sheet feed path 5a or 5d.
Thereafter, similar to the formation of the image on the first surface, an image is formed on a second surface of that sheet P, and then, the sheet is discharged out of the image forming apparatus by the pair of sheet discharge rollers 17. In this way, the image forming apparatus 1 shown in FIG. 1 can form the images on both surfaces of the sheet P.
The image forming apparatus 1 serves to alternately send a sheet P fed from the deck 3a and having a first surface on which an image is to be formed and a sheet fed from the re-feed path 20 and having the first surface on which the image was formed to the image forming portion 11. That is to say, the sheet P having the first surface on which the image is to be formed and the sheet P having the first surface on which the image was formed and the second surface on which the image is to be formed are alternately introduced into the registration introduction path 5b (this is referred to as "alternate feeding" hereinafter).
Accordingly, the flapper 16 can effect the re-feed guide of the sheet having the first surface on which the image was formed and the second surface on which the image is to be formed and the discharge guide of the sheet P having both surfaces on which the images were formed alternately for each sheet.
Incidentally, above the reversing portion 19 and the re-feed path 20, there are disposed cassettes 4b, 4a (stacked vertically) capable of selectively containing large size sheets (such as A3 size sheets) and small size sheets (such as A4 size sheets). For example, the cassettes 4b, 4a can contain about 500 sheets P.
By the way, in case of the above-mentioned conventional image forming apparatus 1, a both-face unit 21 including the reversing portion 19 and the re-feed path 20, and the sheet feed path 5a for the left deck case 3b are designed so that they can be retracted forwardly (toward a front side of the main body) independently.
Thus, if the sheet P is jammed, when the jam treatment is being effected by retracting either the both-face unit 21 or the sheet feed path 5a, the other cannot be retracted for the jam treatment.
That is to say, the conventional image forming apparatus 1 has a disadvantage that the sheet feed path 5a and the both-face unit 21 cannot be retracted simultaneously.
Further, comparing the installation position of the left deck case 3b with the installation position of the right deck case 3b with respect to the waiting position 20p for the sheet P to be re-fed, since the left deck case 3b is more spaced apart from the waiting position than the right deck case, when the alternate feeding is effected by using the left deck case 3b, the stopping time of the re-fed sheet P is increased accordingly.
Accordingly, in the conventional image forming apparatus 1, in the case where the small size sheets (such as A4 size sheets) in the deck case are copied by several hundred sheets, when the sheets P in the right deck case 3a are copied, since the sheets can be copied with substantially the same sheet-to-sheet interval, the both-face copying productivity is good, but, when the sheets P in the left deck case 3b are copied, the sheet-to-sheet interval (sheet interval) is increased, with the result that the both-face copying productivity is worsened.
To solve this problem, it is considered that a conveying speed of the sheet P supplied from the left deck case 3b. In this case, however, it is required that the conveying speed must be increased by several times in order to make the sheet-to-sheet interval for the sheets supplied from the left deck case substantially the same as the sheet-to-sheet interval for the sheets supplied from the right deck case 3a, with the result that control becomes complicated and/or conveying performance becomes unstable.
Accordingly, an object of the present invention is to provide an image forming apparatus in which (1) a re-feed path and a sheet feed path can be retracted simultaneously, and (2) even when a sheet is fed either one of sheet stacking means arranged side by side, a both-face image processing time for the sheet becomes substantially the same, and any loss of productivity does not occur in feed position switching.